In a randomized clinical trial of succimer, an oral chelating agent that lowers blood lead concentration, children with blood leads of 20 to 44 micrograms per deciliter were given succimer or placebo when they were about 2 years old and followed with cognitive and behavioral testing. We reported previously that, despite lower blood lead levels during treatment, children given succimer had test scores that were no different from those of children given placebo at ages five or seven years. We have arranged with CDC to analyze blood samples collected in this project for methyl mercury, inorganic mercury, and cadmium, which will allow a randomized trial of succimer for chelation of these compounds without enrolling any new subjects or collecting any specimens. In data presented at meetings, succimer does not appear to be an efficacious chelating agent for mercury.